In the past, it has been the practice to milk a cow into a receptacle (typically a very large glass jar) allowing the milk yield from the cow to be observed before the milk is transferred to a bulk tank. Currently it has been more usual for the milk to go directly into a suction line which is a pipe of substantial diameter, often over 50 mm diameter, and which extends through the milking parlour and conveys the milk to a bulk tank (so-called direct-to-line milking). When this is done, the milk from a cow may be passed through a milk meter in order to check the milk yield from the cow. This may be done at intervals, perhaps every two weeks, or may be done more frequently, even at every milking.
A milk meter incorporates provision for taking a sample of the flow of milk. The sample is taken as a proportion of the milk as it is coming from the cow so that the sample which is collected should match the overall composition of the milk from the cow even though the composition changes progressively during milking with most of the butter fat coming in the later stages of milking. It is possible to take a sample of the milk at every milking and this is done in research institutions but normal practice in commercial farming is to take samples at intervals. Samples which are taken and sent for analysis are not nearly so large as the samples collected by a milk meter. On days when samples are taken, the task of taking a sample collected by a milk meter, putting a portion of it into a labelled container and putting the remainder back into the collected milk is a substantial addition to the workload.
Milk meters with the facility to collect a sample during milking can be subdivided broadly into manual meters and electronic meters.
Manual milk meters generally operate by dividing the flow of milk from the cow and continuously separating out a fraction of the milk into a sample collecting vessel. The amount which is taken is a fixed proportion of the milk which is flowing from the cow, so the size of the sample which is collected is a known fraction of the total quantity of the milk from the cow, enabling the milk yield of the cow can be calculated from the size of the sample.
Thus, a manual milk meter is actually a device for subdividing the milk flow from a cow and diverting a fixed proportion of that flow to an outlet onto which a sample collecting vessel is attached.
An electronic milk meter is a more elaborate and more expensive device which measures the total weight or total volume of milk from the cow, often by counting how many times the milk fills a space of fixed size. Generally such meters also have the ability to separate out a proportion of the flowing milk into a sample vessel.
Milk meters usually comply with standards laid down by the International Committee for Animal Recording (ICAR). Because of a requirement to obtain a sample with the same composition as the overall milk yield, the sample collecting vessel is typically large enough to hold a sample volume of 0.8 liter. It is normal to take out a small fraction of this relatively large sample for testing and to put the remainder into the bulk tank.
It is possible to take a sample without using a milk meter. Devices are known which can be fitted into the vacuum line from the cluster to the main suction pipe and which will take out a small sample. Such a sample may be less representative of the overall composition of the milk than a (larger) sample taken with a milk meter meeting the ICAR standard.
Depending on the purpose for which the sample is taken, it may not matter whether its composition is representative of the overall composition of the milk from the cow. Analysis of the butter fat content requires the composition of the sample to be a good match to the composition of the overall milk yield, because butter fat is concentrated in the later portions of the milk taken from a cow during milking. However, other constituents which occur in milk such as hormones may be distributed uniformly through the milk and therefore can be detected in any sample.
Since all the milk from a herd of cattle goes into a bulk tank, there is no necessity to clean out the milking machinery after milking one cow and before milking the next. Consequently a sample of the milk taken from one cow may contain traces of milk from the cow previously milked with the same cluster of teat cups.
After milking a herd of cows the pipework in the milking parlour is normally cleaned by a clean-in-place technique in which washing liquors are circulated through the pipework. Most sample collecting vessels must be detached from the milk meters and cleaned separately.
For many years milking parlours have had a number of cow stalls in which cows are milked concurrently. One significant development in recent years is the robotic milking stall which is often a single stall milking unit. This invention is applicable both to a single stall milking unit, as well as a multi stall milking parlour.